1. Field of the Invention
The present invention relates to a device for sensing an oxygen concentration in gaseous body, such as exhaust gas of an internal combustion engine.
2. Description of Background Information
Air-fuel ratio feedback control systems for an internal combustion engine are becoming generally used. They are constructed such that the oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor and an air-fuel ratio of mixture to be supplied to the engine is feedback controlled in response to a result of the detection of the oxygen concentration so as to purify the exhaust gas and improve the fuel economy.
As an example of an oxygen concentration sensing device for use in the air-fuel ratio control system of the above mentioned type, an oxygen concentration sensing device having an output signal whose level is proportional to the oxygen concentration in test gas (whose oxygen concentration is to be measured) is described in Japanese Patent Application laid open No. 58-153155. This oxygen concentration sensing device has a sensor element which has a general construction including a pair of flat solid electrolyte members having oxygen ion permeability. These oxygen-ion conductive solid electrolyte members, operative as active plates, are placed in the atmosphere of the oxygen-containing test gas. Further, two electrodes are provided on the front and back surfaces of both of the solid electrolyte members. In other words, each pair of electrodes sandwiches each solid electrolyte member. These two solid electrolyte members each having a pair of eletrodes are arranged in face to face relation with each other to form a gap portion, or in other words, a restricted region between them.
With this arrangement, one of the solid electrolyte members serves as an oxygen pump element and the other one of the solid electrolyte members serves as a sensor cell element for sensing an oxygen concentration ratio. In the atmosphere of the test gas, a drive current is supplied across the electrodes of the oxygen pump element in such a manner that the electrode facing the gap portion is used as a negative electrode. By the supply of this current, the oxygen component of the gas within the gap portion is ionized on the surface of the negative electrode of the solid electrolyte member operating as the oxygen pump element. The oxygen ions migrate through the inside of the oxygen pump element to the positive electrode, where the oxygen ions are released from the surface of the positive electrode in the form of the oxygen gas.
While this movement of oxygen ions is taking place, an electric potential is generated across the electrodes of the solid electrolyte member operating as the sensor cell element because the oxygen concentration is different for the gas in the gap portion and the gas outside the electrodes of the sensor cell element. This difference of the oxygen concentration is caused by a reduction of the oxygen gas component within the gap portion. Then, if the magnitude of the electric current supplied to the sensor cell element is varied so as to maintain the electric potential across the sensor cell element, the magnitude of the electric current varies substantially linearly in proportion to the oxygen concentration of the test gas at room temperature.
In this type of oxygen concentration sensing device, if an excessive current is supplied to the oxygen pump element, it causes the so called blackening phenomenon by which the oxygen ions are removed from the solid electrolyte members. For instance, when zirconium dioxide (ZrO.sub.2) is utilized as the solid electrolyte, the oxygen ions (O.sub.2) are taken from the zirconium dioxide (ZrO.sub.2) so that zirconium (Zr) is separated out. As a result of this blackening phenomenon, deterioration of the oxygen pump element takes place rapidly, to cause a debasement of an operation of the oxygen concentration sensing device as a whole.
In air/fuel ratio control systems using this type of oxygen concentration sensing device, magnitude of the current to be supplied to the oxygen pump element is set at a level below a critical level of the occurence of the blackening phenomenon in order to prevent the said phenomenon. At the same time, the magnitude of current is determined so that the voltage level of the output signal of the oxygen concentration sensing device becomes equal to a predetermined reference voltage under a condition in which the air/fuel ratio of mixture to be supplied to the engine is equal to a target air/fuel ratio. Therefore, by comparing the output signal level of the oxygen concentration sensing device with the reference voltage, detection is performed as to whether the air/fuel ratio of mixture is on the rich side or the lean side with respect to the target air fuel ratio. If the air/fuel ratio control system is of the type in which the air/fuel ratio is controlled by the supply of secondary air, the secondary air is supplied when the rich air/fuel ratio is detected, and the supply of the secondary air is stopped when the lean air/fuel ratio is detected. In this way, the air/fuel ratio of mixture to be supplied to the engine is controlled toward the target air/fuel ratio.
For supplying the current to the oxygen pump element, an arrangement is generally used in which the magnitude of current flowing through the oxygen pump element, i.e. the pump current, is detected and the supply of the pump current is controlled by a constant current circuit which operates according to a result of comparison between the detected magnitude of the pump current and a reference current value.
FIGS. 1A and 1B show the variation of the control voltage supplied to the constant current circuit and the corresponding variation of the current supplied to the oxygen pump element in a conventional arrangement. As shown in FIG. 1A, when the supply of the control voltage to the constant current circuit is initiated, for instance, at the time of the start of the engine, the constant current circuit starts to supply the pump current to the oxygen pump element. However, due to a delay of response of the air/fuel ratio control system, the pump current does not reach a desired constant level immediately. Instead, as shown in FIG. 1B, an overshoot of the pump current occurs during a transitional period. Therefore, a problem has been experienced that the magnitude of the pump current exceeds the critical level of the occurence of the blackening phenomenon so that the blackening phenomenon actually takes place.
In addition, because of the presence of the gap portion between the oxygen pump element and the sensor cell element, a delay in the response of the sensor cell element inevitably arises. Particularly, the level of the output signal does not increase and reach the reference voltage immediately, even though the pump current to the oxygen pump element has risen above the constant current value corresponding to the reference current value after the start of the supply of the pump current. Instead, the output signal level increases gradually as illustrated in FIG. 1C.
For this reason, although the output signal level of the sensor cell element is monitored for detecting an overcurrent flowing through the oxygen pump element in some systems, it has been difficult to prevent the generation of an overcurrent immediately after the start of the supply of the pump current to the oxygen pump element.